Thursday, December 23, 2010

How diet shaped human evolution

Anyone who is keenly interested in having a better understanding of why we eat what we eat as human beings should take an hour or so to watch this introductory talk given by anthropologist Teresa Steele, of UC Davis, given at the California Academy of Sciences on the topic of evolution of the human diet.

I found her talk fascinating, especially because I've been highly interested in how the use of fire and aquatic animals may have played a part in fueling human brain growth, so I ended up taking copious notes. I should note that there isn't anything new presented here, but Steele is excellent at presenting the chronology. If you don't have an hour to watch, then just see my notes below chapter by chapter from "Australopithecus to agriculture."

Human diet is unique among apes

Steele finds that diet is central to her research. "If we want to live, we have to eat," she says. Food is what ultimately supports demographic populations. One thing that is unique about humans in comparison to other apes is a long childhood, a long learning period, that is required for acquiring the knowledge necessary to become successful foragers in a wide environment. After all, humans have exploited almost every nutrient resource in their short time on the Earth.

Another unique thing is how much meat we consume. A large portion of our calories comes from meat. Unlike chimpanzees, who eat the most meat among apes, human eat about 10 times more, Steele said. And we eat animals that are usually larger than us like wildebeasts, reindeer, and mammoths. Steele shows a graph comparing chimp diets to that of tropical hunter gatherers groups, who typically eat little meat. Other hunter-gatherers of the North like the Inuit eat a diet almost entirely of meat. In general, humans specialize in acquiring nutrient-dense foods meats, tubers, and nuts, while chimps select non-nutrient dense like leaves that are more easily collected.

Research themes

When did these differences evolve? Steele presents us with her research themes, which include the following:

Meat eating. We are consuming animals that are larger than ourselves like wildebeast, reindeer, horses, and so on. Chimpanzees hunt for colobus monkeys, birds, and small amphibians. So when did meat eating appear and when did the transition occur to eating animals larger than us?

Hunting technology. What technology did humans use to acquire large animals? Spears, bows and arrows, projectile technology? These are complex, so they can represent greater cognition. When did they occur?

Intensification of resource use, including agriculture. This happened much more recently.

Methods of Study

What methods does Steele use to construct ancient human diets? She says that zooarchaeology and tool analyses gives us a window into ancient demographies. There are stone, bone and antler tools. And, on occasion, organic wood and plant tools are preserved. Also, biological anthropology helps tell us more such as skeletal morphology and bone chemistry.

Lucy's diet

Steele introduces the diet of Lucy's species first, Austrolopithecus afarensis of 3.7-2.8 mya, who ate a flexible diet suitable for a variety of habitats.

The skeletal biomechanics and dental structure suggest they ate mostly soft fruits and occasional hard seeds. However, Steele says we assume that they may have eaten some meat because chimps eat meat, but it's unclear just how much.

She points out that, recently, there was a groundbreaking discovery published in Nature(and reported in Scientific American by the science writer Kate Wong (Twitter: @katewong) ) of cut-marked bones in Dikika, Ethiopia suggesting Lucy's species even used stone tools for eating meat.

"This has opened up a window," Steele says for more research, especially in the possibility of stone tool use for extracting nutrients from carcasses of smaller animals. It's worth noting that no stone artifacts were found associated with the cut-marked bones (paleoanthropologist John Hawks (Twitter: @johnhawks) has written more about this topic on his blog).

Cut-marked bones 2.5 million years ago

Typically, a discussion of human diet begins at about 2.5 mya when there is an abundance of cut-marked bones (such as the jaw of a wildebeest) and percussion marks from marrow extraction. Marrow has been an important human resource for nutrients up until modern times because it's high in fat, high in calories.

There is also evidence of Oldowan artifacts (hominin stone tools) available so we know what they were using to get to the marrow.

Then, at about 1.8 mya there are a lot more assemblages, more stone tools, as found in Olduvai Gorge, Tanzania, by Mary Leaky. There are also lots of large bodies bovids and carnivores on the landscape. Steele asks, How did these ancient hominids acquire these large carcasses? Is it conceivable that they could've brought down a wildebeast with just tools?

This is where we get into a discussion of scavenging versus hunting, she said. A related discussion is what percentage of the diet was meat-based versus plant-based. Also, were these ancient hominins practicing passive scavenging getting to a carcass to get the last scraps of meat or breaking open bones for marrow. Or was it active scavenging, chasing off carnivores?

These are all active areas of research. For answers, researchers look in locations of lakeside margins. Bovids came to drink, carnivores know this, we look into these locations to try and reconstruct the foraging.

Aquatic animals

Published recently in the springtime, was a paper suggesting that 1.9 mya in East Turkana, there's evidence of Oldowan foraging of carcasses of aquatic animals like crocodiles and turtles. Steele shows a cut marks on a toe bone of a croc, turtle shells and catfish bones.

"For the first time, we see exploitation of aquatic resources highlighting the diversity of diet. Hominins are very opportunistic, exploiting whatever was available," Steele said.

"This also raises a challenge as with cut-marked bones with Dekika, to try to see if there are cut-marks on similar bones," Steele explains. "The small animal component has been overlooked so we may need to look closer."

Steele also discusses another interesting aspect of using aquatic resources (which will interest any nutritionist like myself). The aquatic resources would have been an easier way to access long-chain omega-3 fatty acids, which are also present in organ meats and brain tissues of large animals.

"The long-chain unsaturated fatty acids are needed for brain growth," she explained. "At this time period we do see an expansion of brain sizes, so perhaps there's a relationship here. We need more data, more examples where we see brain expansion with this kind of diet."

Archeulean hunting and scavenging

Moving more recently in time, we see Homo erectus, hominins of larger body size, and who were first to populate Eurasia 1.6 mya to 285 kya. Were they hunting or actively scavenging? This is unclear, but earlier in Archeulan, we see evolution of technology.

Tear-drop shaped hand axes appear and body size changes. The humans are obviously living in social groups. An illustration she uses takes the liberty of showing piles of plant remains used to make wooden spears. The plant use is unknown.

There are a large number of animal bones with few cut marks. So, the question remains, were hominins still minor players as carnivores, simply cutting off limbs and eating elsewhere. The challenge is finding places away from water sites such as in caves.

Also, we start asking questions about use of fire at this time period.

Wood spears

At around 400 kya, Steele shares that there are one or two examples of exceptional preservation of organic materials such as wooden spears (survived in an oxygen-poor environments from marshes of Germany). They are more likely to be thrusting spears. They have been fire-hardened, sharpened, so it indicates use of fire.

Fire is really useful for warmth, protection from predators, for cooking and cooking really changes the nature of food. It helps make inedible foods edible, releases nutrients for our digestive systems. But fire doesn't preserve well.

The earliest known site where fire is documented is in Israel, dated to 780 kya. "We have an indicator of fire use and plant remains. They're preserve better once charred in archaeological sites," Steele says. "We don't find it common until about 300,000 years ago." This is between Oldowan and modern behavior in the Archeulian.

Neandertals

About 200 kya came the Neandertals and they were competent hunters and manufacturers of stone tools. Interestingly, despite these complex behaviors, they did not have as long a childhood. The Neandertals were able to pick up their abilities pretty early in life.

As part of her post-doc in Germany at Max Plank Institute, Steele worked with identifying species in archaeological sites where Neandertals hunted reindeer and bison. She showed antlers, elbows of reindeer fractured for extracting marrow, and examples of bones in discard piles due to little meat.

"We also see very little carnivore involvement and abundant human impacts, unlike the earlier where there was very heavy carnivore involvement meaning humans were hunting," she said. The Neandertals were dominant carnivores by this time.

Now we can ask about hunting strategy. Steele explains she uses a very low tech method: "We have a number of mandibles, so just looking at the eruption of teeth, we can reconstruct ages of animals." Also, reindeer are conveniently sexually dimorphic and because reindeer give birth at a moment in spring (babies are born at once) we can look at eruption of teeth to see if they're hunted. In a specific location, all ages are present, males and females, so it looks like the reindeer herd would have been slowed allowing the humans to hunt more of them.

Bone chemistry

Carbon isotopes tell us about the vegetation in the environment and nitrogen isotopes tell us about the trophic levels. Carnivores have more concentration of nitrogen. Animals that are aquatic even more nitrogen, so we can look at bone chemistry to reconstruct diet. There aren't much indicators of plant remains, but in a Neandertal tooth you see it's heavily etched by roots because of the acid of roots. The bone chemistry data put Neandertals right along the lines of other carnivores. The majority of protein came from meat (although not mentioned in the talk, new findings show they also practiced cannibalism, reported via science writer Carl Zimmer (Twitter: @carlzimmer)).

Hunting technology

How were the Neandertals doing the hunting? It appears they were using thrusting spears. We know this because it's possible to look at stone artifacts to see if they are aerodynamic or more asymetrical and lumpy for a thrusting spear. We can look at the breakage of the tip as well as the butt. In characteristic way we can look at the breakage.

Middle stone age in Africa 285,000

So while Neandertals are doing their thing in Europe, what's going on in Africa? In Africa, we have the middle stone age and humans who were morphologically similar to us. The big discussion in paleoanthropology is, How modern were they? Did they have symbolism? Were they just like us or behave more like Neandertals without as much symbolism?

In the middle stone age we have good evidence of hunting and burning. There was abundant burning. But, within the middle stone age, we see no evidence of consumption of fish. The people seem to be limited in capturing fish and birds, although there were people accessing coastal resources along the southern coast of Africa, eating a number of mollusks. Could mollusks have fueled brain growth and brought with it symbolic behavior? There were also a number of fireplaces. Did fire fuel brain growth (if you ask primatologist Richard Wrangham as I did last February, then the answer is a resounding "yes!")? This is something that requires further research.

Modern humans in Europe

In Europe about 40 to 10 kya, we have Upper Paleolithic with fully modern humans in Europe. They hunted large game similar to Neandertals and with projectile technology unlike Neandertals. People who were just like us in biology and behavior. This is when we see projectiles for the first time. We see the reconstruction of a spear thrower, with an adle addle.

These modern humans then also enjoyed a diverse diet with abundant small game like fish and flying birds. That's quite different than what their Neandertals cousins were doing, and what humans in Africa of the middle-stone age were doing.

We can also see this in the bone chemistry of the Upper Paleolithic humans. There was definitely protein coming in from aquatic sources, per the nitrogen values in the bones. It's also clear from the bone chemistry that modern humans were eating a much more diverse diet.

Plant use

Getting back to plant use, just recently in PNAS, an article was published about use of plants in Paleolithic times. Grindstones and pestles were used to grind starch grains, reeds, cattailes and ferns that have underground storage organs (roots). These grindstones pulverized the roots and perhaps made flour out of them. So, this is it, the diversity of diet that spread from Africa about 50 kya, and support for the hypothesis that humans replaced Neandertals because of flexibility of diet. Is this what allowed humans to be more successful?

Intensification of resource extraction, including agriculture

Bringing us into more recent time period to complete the story, 50kya humans colonized Europe and Asia and Australia. At around 15kya, they colonized the new world. So, by 10kya we have humans everywheere by 10kya other than Pacific islands and Antarctica. Diet tends to evolve and change. Humans don't stay focused on large game, and birds and fish. They intensify. What we see with intensification in the Holocene is the use of technology to extract nutrients from resources.

Steele shows pictures of mussel shells having accumulated over a short period of time. There was a heavier investment in technology. This creates a stable food supply that allows populations to grow. "We can see this in our local California native indians," she said. Just to highlight investment in technology, she shows slides on the natives' use of technology. "These are all the steps to take acorns and make it into something consumable. They're toxic, so you have to dry them, pulverize and leach them. It requires very heavy technological input."

The intensification brings with it the origins of agriculture at 10 kya. At 10kya we see changes in environment tha promote plant resources, a shift in global climate where there's more CO2, a more wet and stable environment, more admittable to plant production. People are becoming more dependent on smaller resources from agriculture. The fish, they help populations to grow and hunter-gatherer populations are more stable. It's clear from her slide that because of agriculture, there's an uptick in human population growth. Then, when industrialized agriculture arrives, there's an inflection point when we see a high rate of population growth. That's where we are today in the evolution of human diets. That's 4 million years (in 40 minutes).

Question 1: Why did humans replaced Neandertals?

The first question posed to Steele after her talk was about her thoughts were about why humans replaced Neandertals. She answered, "Yes, I think ultimately it's due to dietary differences." There's not much differences in species hunted, not so different butchery, but you do see a difference in stone artifacts and projectile points. The modern human tools were more reliable and accurate. They would've been able to obtain a larger number of reindeer, and been more consistent in hunting, along with having a more diverse diet.

The more ultimate explanation, however, was if it was cultural. Did modern humans have a more complex language? Could symbolism have allowed us to communicate in a more effective way, made our hunting more effective, that's where we're going now with the research. Language is fundamental, so if we can track where language evolved, then we'll find more answers?

Question 2: What conclusive evidence is there of cut marks?

The question asked to Steele reverted back 3.2 mya to how solid the evidence was of Australopithecus afarensis making cut marks. Steele answers that the cut marks are just as conclusive as later time periods. "If we are going to accept the later cut marks, then we have to accept the earlier," she said. "For me they're fine in terms of more recent assemblages. The challenge is to find more cut marks to see if it was widespread or a one-time thing. Who made them? Where are the stone tools?" That's the next project.

Question 3: What ratio of fatty acids in diet correspond to brain size?

Lastly, an audience member asked if recent work on long-chain omega-3s on mood disorders supports the theory that omega-3s from aquatic resources fueled brain growth. The quiestoner also mentions work by others on omega-3 to omega-6 ratios, which has changed since huntergatherer times (from 1:1-3 to 1:10 to 1:20). Could this be the reason that brain sizes are getting smaller?

Steele answers that, in general, there's body size reduction and brain size reduction. Hunter-gatherers of the anthropological record were quite robust. Now we see decrease in stature, brain size reducing, body size reducing. The change in body shape may be due to changes in diet. Whether it's omega-3/omega-6? Steele says she couldn't say for sure if that's the case.

(Note: Hat tip to @KeithNorris and @evolvify (see blog post here) for first alerting me to this new video via their tweets).